Control of humidifier chamber temperature for accurate humidity control

ABSTRACT

A breathing assistance system for delivering a stream of heated, humidified gases to a user, comprising a humidifier unit which holds and heats a volume of water, and which in use receives a flow of gases from a gases source via an inlet port, the flow of gases passing through the humidifier and exiting via an exit port, the system further having a temperature sensor which measures the temperature of the gases exiting the humidifier unit, an ambient temperature sensor which measures the temperature of gases before they enter the humidifier unit, and a flow sensor which measures the flow rate of the gases stream, the system also having a controller which receives data from the temperature and flow sensors, and which determines a control output in response, the control output adjusting the power to the humidifier unit to achieve a desired output at the humidifier unit exit port.

FIELD OF THE INVENTION

This invention relates to methods and apparatus for controlling thehumidity level and flow rate of gases in a device that provides a streamof heated, humidified gases to a user for therapeutic purposes. Thisinvention particularly relates to methods and apparatus for controllingthe humidity of a gases stream in devices that provide humidified airfor: respiratory humidification therapy, high-flow oxygen therapy, CPAPtherapy, Bi-PAP therapy, OPAP therapy, etc, or humidification of gasesused for insufflation or keyhole surgery.

BACKGROUND

Devices or systems for providing a humidified gases flow to a patientfor therapeutic purposes are well known in the art. Systems forproviding therapy of this type (for example respiratory humidification)have a structure where gases are delivered to a humidifier chamber froma gases source. As the gases pass over the hot water, or through theheated, humidified air in the humidifier chamber, they become saturatedwith water vapour. The heated humidified gases are then delivered to auser or patient downstream from the humidifier chamber, via a gasesconduit and a user interface. The gases delivery system can be a modularsystem that has been assembled from separate units, with the gasessource being an assisted breathing unit or blower unit. That is, thehumidifier chamber/heater and the blower unit are separate (modular)items. The modules are in use connected in series via connectionconduits to allow gases to pass from the blower unit to the humidifierunit. Alternatively, the breathing assistance apparatus can be anintegrated system, where the blower unit and the humidifier unit arecontained within the same housing in use. In both modular and integratedsystems, the gases provided by the blower unit are generally sourcedfrom the surrounding atmosphere. A third general form of breathingassistance system, which is typically used in hospitals, is one wherethe breathing assistance system receives at least a portion of the gaseswhich it uses from a central gases source, typically external to thearea of use (e.g. a hospital room). A gases conduit or similar isconnected between an inlet which is mounted e.g. in the wall of apatients room (or similar). The gases conduit is either connecteddirectly to the humidifier chamber in use, or a step-down control unitor similar can be connected in series between the gases inlet and thehumidifier chamber if required. This type of breathing assistance systemis generally used where a patient or user may require oxygen therapy,with the oxygen supplied from the central gases source. It is common forthe pure oxygen from the gases source to be blended with atmospheric airbefore delivery to the patient or user, for example by using a venturilocated in the step-down control unit. In systems of the type where atleast some of the gases are delivered from a central source, there is noneed for a separate flow generator or blower—the gases are deliveredfrom the inlet under pressure, with the step down control unit alteringthe pressure and flow to the required level.

An example of a known, prior art, type of modular system usingatmospheric gases only is shown in FIG. 1.

In typical integrated and modular systems, the atmospheric gases aresucked in or otherwise enter a main ‘blower’ or assisted breathing unit,which provides a gases flow at it's outlet. The blower unit and thehumidifier unit are mated with or otherwise rigidly connected to theblower unit. For example, the humidifier unit is mated to the blowerunit by a slide-on or push connection, which ensures that the humidifierunit is rigidly connected to and held firmly in place on the main blowerunit. An example of a system of this type is the Fisher and PaykelHealthcare ‘slide-on’ water chamber system shown and described in U.S.Pat. No. 7,111,624. A variation of this design is a slide-on or clip-ondesign where the chamber is enclosed inside a portion of the integratedunit in use. An example of this type of design is described in WO2004/112873.

One of the problems that has been encountered with systems that providea flow of heated, humidified gases to a patient via a gases conduit andan interface is that of adequately controlling the characteristics ofthe gas. Clearly, it is desirable to deliver the gas to the patient(i.e. as it exits the user interface) with the gas at precisely theright temperature, humidity, flow, and oxygen fraction (if the patientis undergoing oxygen therapy) to provide the required therapy. A therapyregime can become ineffective if the gases are not delivered to thepatient with the correct or required characteristics. Often, the mostdesirable situation is to deliver gases that are fully saturated withwater vapour (i.e. at substantially 100% relative humidity) to a user,at a constant flow rate. Other types or variations of therapy regime maycall for less than 100% relative humidity. Breathing circuits are notsteady-state systems, and it is difficult to ensure the gases aredelivered to a user with substantially the correct characteristics. Itcan be difficult to achieve this result over a range of ambienttemperatures, ambient humidity levels, and a range of gas flows at thepoint of delivery. The temperature, flow rate and humidity of a gasesstream are all interdependent characteristics. When one characteristicchanges, the others will also change. A number of external variables canaffect the gases within a breathing circuit and make it difficult todeliver the gases to the user at substantially the right temperature,flow rate and humidity. As one example, the delivery conduit between thepatient or user and the humidifier outlet is exposed to ambientatmospheric conditions, and cooling of the heated, humidified gaseswithin the conduit can occur as the gas travels along the conduitbetween the exit port of the humidifier chamber and the user interface.This cooling can lead to ‘rain-out’ within the conduit (that is,condensate forming on the inner surface of the conduit). Rain-out isextremely undesirable for reasons that are explained in detail in WO01/13981.

In order to assist in achieving delivery of the gases stream with thegases having the desired characteristics, prior art systems have usedsensors (e.g. temperature and humidity sensors) located at variouspositions throughout the breathing circuit. Thermistors are generallyused as temperature sensors, as these are reliable and inexpensive.Humidity sensors such as the one described in U.S. Pat. No. 6,895,803are suitable for use with systems that deliver heated humidified gasesto a user for therapeutic purposes.

In order to achieve delivery of the gases to the patient at the correcttemperature and humidity, it is necessary either to measure or sense thegases characteristics at the point of delivery, or to calculate orestimate the gases characteristics at the point of delivery frommeasurements taken from elsewhere in the system. In order to directlymeasure the gases parameters at the point of delivery, sensors must belocated at or close to the point of delivery—either at the end of thepatient conduit or within the interface. Sensors located at or close tothe point of gases delivery will give the most accurate indication ofthe gases state. However, one consideration when designing a breathingcircuit is to ensure that the components used in the breathing circuitcan be repeatedly connected and disconnected to and from each other,with high reliability. Another consideration is to keep the weightcarried by the patient in use to a minimum, and therefore it isdesirable to keep the number of sensors at the patient end of theconduit to a minimum, or remove the need for these altogether. It isalso desirable to keep the total number of sensors in the system to aminimum, in order to reduce costs and complexity (e.g. an increasednumber of electrical and pneumatic connections).

In order to overcome or sidestep the problem or trade-off of accuratemeasurement of the gases characteristics vs complexity vs cost vs weightcarried by the patient vs reliability, sensors can be located at variousother points within the system to measure the parameters of the gas atthose points, and the readings from these sensors can be used by acontroller to estimate or calculate the characteristics of the gases atthe point of delivery. The controller then adjusts the output parametersof the system (e.g. fan speed, power to the humidifier chamber heaterplate, etc) accordingly. One example of a system and method where thistype of calculation is carried out is disclosed in WO 2001/13981, whichdescribes an apparatus where there are no sensors at the patient end ofthe conduit. A, temperature sensor is located proximal to the heaterplate in order to measure the heater plate temperature. The flow ofgases through the humidifier chamber is estimated, and the appropriatepower level for the heater plate is then determined by a centralcontroller. The controller estimates the power supply to the heaterhumidifier plate, and the power required by the conduit heater wire forachieving optimal temperature and humidity of the gases delivered to apatient.

One possible disadvantage of systems and methods which estimate thegases characteristics (such as the system and method disclosed in WO2001/13981) is that the estimations and algorithms used are not asaccurate as is necessary. There are many variable factors that candetrimentally effect the accuracy of the calculation algorithms used bythe controller. These factors may not have been taken into considerationwhen the algorithm was designed. For example, the apparatus and inparticular the humidifier chamber can be subject to convective heat loss(‘draft’) which is created by external airflows, particularly inventilated spaces. The flow velocities of the air vary in magnitude,direction and fluctuation frequency. Mean air velocities from below 0.05m/s up to 0.6 m/s, turbulence intensities from less than 10% up to 70%,and frequency of velocity fluctuations as high as 2 Hz that contributeup to 90% of the measured standard deviations of fluctuating velocityhave been identified in the occupied zone of rooms—for one example, seeVolume 13, number 6 of HVAC&R Research—paper tided: ‘accuracylimitations for low velocity measurements and draft assessment inrooms’, by A Melikov, Z Popiolek, and M. C. G. Silva.

The system disclosed in WO 2001/13981 is unlikely to be able to providethe control precision necessary to control humidity accurately withoutsubstantial rainout occurring. A user or manufacturer may be forced totrade-off delivery of gases at a lower humidity level, against anincreased possibility of rain-out, against the number of sensors usedand their location in the breathing circuit. For example, when theincoming gas delivered to the humidifier chamber from the compressor orblower (particularly in an integrated blower/humidifier breathingassistance system) has an increased temperature, the chamber temperatureshould be accurately compensated to achieve the desired dew point. Ifthe air coming into the chamber is warm and the air temperature isincreasing with an increase in flow, then the inaccuracy of a setcalculation algorithm will increase.

It should further be noted that prior art systems frequentlymeasure/calculate and display the humidifier chamber outlet temperature.Displaying the temperature reading is often inadequate for a user tomake an informed decision, as the temperature does not always directlyrelate to the gases humidity state. This is due to a number of factors,of which the following are examples, but not an exhaustive list.

-   -   1. High temperature of the incoming gas.    -   2. Very low or very high flow rate.    -   3. Cooling of the humidifier chamber by convection of the        ambient air around the humidification chamber.    -   4. Mixing of outgoing and incoming gases inside the chamber.    -   5. Condensation of water at the chamber wall or connection tubes        particularly at low ambient temperature conditions.    -   6. Problems with accurate temperature measurements at high        humidity (the ‘wet bulb’ effect).    -   7. Variations in the level of the humidity of the incoming gas.

Furthermore, a user may not always require gases warmed to bodytemperature and 100% humidity. A specific therapy regime may call for ahigh or 100% humidity level, but this can be undesirable for users whouse a mask, as the conditioned gas with high humidity can feeluncomfortable for a user on their skin.

A further problem in system of this type can be outlined as follows: Itis normal in systems such as those outlined above for the fan speed(modular and integrated units) or pressure/flow level (hospital, remotesource units) to be set to a constant level, with the assumption thatthis will provide a constant flow rate throughout the system (oralternatively, if using a central gases source in the system, the flowrate of the incoming gases from the remote source is assumed to remainconstant). A constant flow rate is desirable for the same or similarreasons as outlined above. A constant flow rate is also very desirablewhen using additional or supplementary oxygen, blending this withatmospheric gases. A constant flow rate will help to keep the oxygenfraction at the desired level.

As the gases characteristics are interdependent, a change in the flowrate may lead to a significant change in the humidity, temperature oroxygen fraction of the gases delivered to a user. However, the flowthrough the system may be affected by a number of differentinterdependent variables which are independent of the gases source (e.g.the speed of the fan). These can include increased (or decreased)resistance to flow caused by changes in the position of the userinterface on a user, changes in the way the delivery conduit is bent inuse, etc. The flow rate will also change if, for example, the interfaceis changed to a different size or shape of interface, or a differenttype of interface altogether.

There is therefore a need for a system and method which providesincreased control precision for controlling the humidity, ortemperature, or both, of the gases flow, while at the same timedelivering gases to a patient at the correct temperature, humidity andpressure for effective therapy. There is also the need for a systemwhich compensates for changes in the resistance to flow through thesystem during use in order to provide a substantially constant flow rateat the desired level.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an integratedblower/humidifier system that goes some way towards overcoming the abovedisadvantages, or which provides users with a useful choice.

In a first aspect the invention may broadly be said to consist in abreathing assistance system for delivering a stream of heated,humidified gases to a user for therapeutic purposes, comprising;

a humidifier unit that has an inlet port and an exit port, saidhumidifier unit adapted to in use receive a flow of gases from a gasessource via said inlet port, said humidifier unit further adapted to holdand heat a volume of water in use, in use said flow of gases passingthrough said humidifier unit and becoming heated and humidified, saidheated humidified gases exiting said humidifier unit via said humidifierunit exit port,

an exit port temperature sensor adapted to measure the temperature ofgases exiting said humidifier unit,

an ambient temperature sensor adapted to measure the temperature ofgases at a point before said gases enter said humidifier unit,

a flow sensor adapted to measure the actual flow rate of said gasesstream through said system,

a controller adapted to receive data from said ambient temperaturesensor relating to the measured temperature, data from said exit porttemperature sensor relating to the measured temperature, and data fromsaid flow sensor relating to said actual flow rate, said controllerdetermining a control output in response, said control output adjustingthe power to said humidifier unit to achieve a desired output at saidhumidifier unit exit port.

In a second aspect the invention may broadly be said to consist in abreathing assistance system for delivering a stream of heated,humidified gases to a user for therapeutic purposes, comprising;

a humidifier unit that has an inlet port and an exit port, saidhumidifier unit adapted to in use receive a flow of gases from a gasessource via said inlet port, said humidifier unit further adapted to holdand heat a volume of water in use, in use said flow of gases passingthrough said humidifier unit and becoming heated and humidified, saidheated humidified gases exiting said humidifier unit via said humidifierunit exit port,

a delivery conduit and user interface configured to in use receive saidheated humidified gases from said exit port for delivery to said user,said delivery conduit having a heater wire adapted to heat the gaseswithin said conduit,

a patient end temperature sensor adapted to measure the temperature ofsaid gases flow at or close to said patient,

a flow probe adapted to measure the actual flow rate of said gasesstream through said system,

said breathing assistance system further comprising a controller adaptedto receive data from said patient end temperature sensor relating to themeasured temperature, and data from said flow probe relating to saidactual flow rate, said controller determining a control output inresponse, said control output adjusting the power to at least saidheater wire to maintain or alter the temperature of said flow of gaseswithin said conduit to achieve a desired patient end temperature andabsolute humidity at said interface.

Preferably said control output relates to a target temperature at saidexit port for a given flow level, and said desired output is a targettemperature, said control output adjusting said power to said humidifierunit to match said measured temperature at said exit port with saidtarget temperature.

Preferably said control output is determined from a rule-based systemloaded in said controller.

Alternatively said control output is determined from at least onemathematical formula loaded in said controller.

Alternatively said control output is determined from a look-up tableloaded in said controller.

Preferably said desired output is a target dew point temperature.

Preferably said target dew point temperature is in the range 31-39° C.

Preferably said user set target dew point temperature provides anabsolute humidity level of substantially 44 mg H₂O/litre of air.

Alternatively said desired output is a target absolute humidity.

Alternatively said desired output is a target temperature and relativehumidity.

Preferably said breathing assistance system also has user controlsadapted to enable a user to set a desired user-set flow rate of gasesthrough said system.

Preferably said breathing assistance apparatus further comprises acontrol unit adapted to in use receive a flow of gases from a remotecentral source, said control unit located in the gases path between saidcentral source and said humidifier unit, said control unit receivingsaid flow of gases and passing said flow on to said humidifier unit viaa gases connection path between said humidifier unit and said controlunit, said user controls adapted to enable a user to set a desireduser-set flow rate through said control unit.

Preferably said control unit further comprises a venturi adapted to mixsaid flow of gases from said central source with atmospheric gasesbefore passing these to said humidifier unit.

Preferably said gases source is a blower unit fluidically connected inuse to said humidifier unit, said blower unit having an adjustable,variable speed fan unit adapted to deliver said flow of gases over arange of flow rates to said humidifier unit and user controls adapted toenable a user to set a desired user-set flow rate, said controlleradapted to control the power to said blower unit to produce saiduser-set flow rate.

Preferably said humidifier unit is a humidifier chamber having a heaterbase, and said breathing assistance system further has a heater plateadapted to heat the contents of said humidifier chamber by providingheat energy to said heater base,

-   -   said breathing assistance system further having a heater plate        temperature sensor adapted to measure the temperature of said        heater plate and provide this temperature measurement to said        controller, said controller determining said control output by        assessing all of said measured temperatures and said measured        flow rate.

Preferably if a target value of said chamber gases outlet temperature isreached and the corresponding heater plate temperature is higher than aset value stored in the memory of said controller for a given pre-settime period, said controller assesses that said humidifier unit isexperiencing high convective heat loss and determines said controloutput according to an altered or different rule set, mathematicalformula or look-up table.

Preferably said controller is further adapted to measure the power drawnby said heater plate for a given pre-set time period and if the powerdrawn is higher than a value stored in the memory of said controller,said controller assesses that said humidifier unit is experiencing highconvective heat loss and determines said control output according to analtered or different control algorithm, mathematical formula or look-uptable.

Preferably said controller is further adapted to measure the power drawnby said heater plate for a given pre-set time period and compare this toa pre-stored set of values stored in the memory of said controller, saidcontroller applying an inversely linear correction factor if themeasured power drawn is not substantially similar to said pre-stored setof values.

Preferably said measured data values and said stored data values must bewithin +/−2%.

Preferably said ambient temperature sensor is located at or close tosaid inlet port to measure the temperature of gases substantially asthey enter said humidifier unit.

Alternatively said ambient temperature sensor is adapted to measure thepre-entry temperature of gases substantially as they enter saidbreathing assistance system, said controller applying a correctionfactor to said pre-entry temperature.

Preferably said controller is adapted to receive at least said user setflow rate and said actual flow rate data from said flow probe or flowsensor, said controller having a coarse control parameters and finecontrol parameters, said controller comparing said user set flow rateand said actual flow rate, said controller using said fine controlparameters to adjust the output of said fan to match said actual flowrate to said user set flow rate as long as said actual flow rate matchessaid user set flow rate to within a tolerance, the value of saidtolerance stored within said controller, said controller using saidcoarse control parameters to adjust the output of said fan to match saidactual flow rate to said user set flow rate if the difference betweensaid user set flow rate and said actual flow rate is outside saidtolerance.

Preferably said coarse control parameters are a first P.I.D. filter andsaid fine control parameters are a second P.I.D. filter.

Alternatively said controller further comprises a compensation filter, alow pass filter, a high pass filter, and a P.I.D. filter, said signalindicative of actual flow rate from said flow probe passed in parallelthrough said low pass filter and said high pass filter, said low passfilter producing a low pass output signal, said high pass filterproducing a high pass output signal that is passed through saidcompensation filter, said low pass output signal subtracted from saiduser set flow rate signal and passed into said P.I.D filter, the outputsignal from said P.I.D filter and the output signal from saidcompensation filter summed and compared to said user set flow rate, saidcontroller using said coarse control parameters to adjust the output ofsaid fan to match said actual flow rate to said user set flow rate ifthe difference between the sum of said output signals and the user setflow rate is outside a pre-set tolerance contained in the memory of saidcontroller.

Alternatively said controller is adapted to receive at least said userset flow rate and said actual flow rate data from said flow probe, saidcontroller having a coarse control parameters and fine controlparameters, said controller comparing said user set flow rate and saidactual flow rate, said controller using said fine control parameters toadjust the output of said fan to match said actual flow rate to saiduser set flow rate as long as said actual flow rate matches said userset flow rate to within a tolerance, the value of said tolerance storedwithin said controller, said controller using said coarse controlparameters to adjust the output of said fan to match said actual flowrate to said user set flow rate if the difference between said user setflow rate and said actual flow rate is outside said tolerance.

Alternatively said controller further comprises a compensation filter, alow pass filter, a high pass filter, and a P.I.D. filter, said signalindicative of actual flow rate from said flow probe passed in parallelthrough said low pass filter and said high pass filter, said low passfilter producing a low pass output signal, said high pass filterproducing a high pass output signal that is passed through saidcompensation filter, said low pass output signal subtracted from saiduser set flow rate signal and passed into said P.I.D filter, the outputsignal from said P.I.D filter and the output signal from saidcompensation filter summed and compared to said user set flow rate, saidcontroller using said coarse control parameters to adjust the output ofsaid fan to match said actual flow rate to said user set flow rate ifthe difference between the sum of said output signals and the user setflow rate is outside a pre-set tolerance contained in the memory of saidcontroller.

Preferably said controller also includes a feedback signal from said fanto said compensation filter so that the input signal to said fan unitcomprises the output signal from said P.I.D. filter and the outputsignal from said compensation filter.

Preferably said actual flow rate data is measured by said at least oneflow probe, said actual flow rate data subtracted from said user setflow data and a signal indicative of the difference sent to both saidfirst and second P.I.D. filters, said controller using either the outputof said first P.I.D. filter or said second P.I.D. filter to adjust theoutput of said fan to match said actual flow rate to said user set flowrate.

Preferably said flow rate is sampled at a sample rate of between 20 and30 Hz.

Even more preferably said sample rate is 25 Hz.

Preferably said actual flow rate data is passed through a first low-passfilter before being subtracted from said user set flow data.

Preferably said first low-pass filter has a cut-off frequency highenough to allow intra breath flow variation to pass unattenuated.

Preferably said actual flow rate data is also passed through anaveraging filter.

Preferably said averaging filter is a second low pass filter.

Preferably the output of said averaging filter is fed back to saidcontroller in place of said direct flow data from said flow probe.

Preferably said controller receives said averaged flow from saidaveraging filter and compares this to said user set flow rate, saidcontroller using coarse control parameters to adjust the flow rate tosaid user set rate if the difference between said user set flow rate andsaid actual flow rate is outside a tolerance value stored in the memoryof said controller, said controller using fine control parameters ifsaid difference is inside said tolerance.

Preferably said tolerance is 3 L/min.

Alternatively said tolerance is variable, and is a percentage value ofsaid actual flow rate as measured by said flow probe.

Preferably said percentage value is between 1-3%.

Alternatively said percentage value is between 3-5%.

Alternatively said percentage value is between 5-7%.

Alternatively said percentage value is between 7-10%.

Preferably said control unit is particularly adapted to receive oxygenas said gases from said remote source, said at least one flow probeadapted to measure said flow rate of gases received from said remotesource and pass said flow rate measurement on to said controller, saidcontroller adapted to determine the flow rate of said gases fromatmosphere based on the known system dimensions, said controllerdetermining the fraction of oxygen in said blended air from said flowrate and said system dimensions.

Preferably said control unit is adapted to receive oxygen as said gasesfrom said remote source, said at least one flow probe adapted to measuresaid flow rate of gases received from said remote source, said systemfurther comprising a second flow probe adapted to measure the flow rateof said gases received from atmosphere, said controller determining thefraction of oxygen in said blended air from said flow rates.

Preferably said system is adapted so that when a user alters said userset flow rate this alters said oxygen fraction.

Preferably said system further has a display adapted to show chamberoutlet dcw point temperature.

Alternatively said display is adapted to show the absolute humiditylevel of gases exiting said chamber.

Alternatively said display is adapted to show absolute humidity andchamber outlet dew point temperature.

Preferably said breathing assistance system also has a humidity sensoradapted to measure the humidity of atmospheric gases entering saidbreathing assistance system, said controller receiving data relating tothe measured humidity,

-   -   said controller determining said control output by also using        said data relating to the measured humidity.

Preferably said system also has a pressure sensor adapted to measure thepressure of atmospheric gases entering said breathing assistance system,said controller receiving data relating to the measured pressure,

-   -   said controller determining said control output by also using        said data relating to the measured pressure.

Preferably said system further comprises a delivery conduit and userinterface configured to in use receive said heated humidified gases fromsaid exit port for delivery to said user, said delivery conduit having aheater wire adapted to heat the gases within said conduit.

Preferably said breathing assistance system further has a patient endtemperature sensor adapted to measure the temperature of said gases flowat or close to said patient, the measured patient end temperature fedback to said controller, said controller adjusting the power to saidheater wire to maintain the temperature of said flow of gases withinsaid conduit.

Preferably said controller receives said measured patient endtemperature data, said controller determining said control output byalso using said data relating to the measured patient end temperaturedata.

Preferably said controller is further adapted to measure the power drawnby said heater wire for a given pre-set time period, and if said powerdrawn by said heater wire is higher than a value stored in the memory ofsaid controller, said controller assesses that said humidifier unit isexperiencing high convective heat loss and determines said controloutput according to an altered or different rule set, mathematicalformula, or look-up table.

This invention may also be said broadly to consist in the parts,elements and features referred to or indicated in the specification ofthe application, individually or collectively, and any or allcombinations of any two or more of said parts, elements or features, andwhere specific integers are mentioned herein which have knownequivalents in the art to which this invention relates, such knownequivalents are deemed to be incorporated herein as if individually setforth.

The term ‘comprising’ as used in this specification means ‘consisting atleast in part of’, that is to say when interpreting statements in thisspecification which include that term, the features, prefaced by thatterm in each statement, all need to be present but other features canalso be present.

BRIEF DESCRIPTION OF THE DRAWINGS

One preferred form of the present invention will now be described withreference to the accompanying drawings in which:

FIG. 1 shows a schematic view of a user receiving humidified air from amodular blower/humidifier breathing assistance system of a known, priorart, type.

FIG. 2a shows a schematic view of a user receiving humidified air fromone variant of the present invention, with the user wearing a nasal maskand receiving air from a modular blower/humidifier breathing assistancesystem.

FIG. 2b shows a schematic view of a user receiving humidified air fromanother variant of the present invention, where the user is wearing anasal cannula and receiving air from a modular blower/humidifierbreathing assistance system.

FIG. 3 shows a schematic view of a user receiving humidified air fromanother variant of the present invention, where the user is wearing anasal mask and receiving air from an integrated blower/humidifierbreathing assistance system.

FIG. 4 shows a schematic view of a user receiving humidified air fromanother variant of the present invention, where the user is wearing anasal cannula, the breathing assistance system receiving gases from acentral source via a wall inlet and providing these to a control unit,which provides the gases to a humidifier chamber in line with anddownstream of the control unit.

FIG. 5 shows a graphical representation of a data set for use with thebreathing assistance system of FIG. 2 or 3, the graph showing curvesrepresentative of seven different constant flow rates over a range ofambient atmospheric temperatures, and a range of target temperatures fora given flow and ambient temperature, the data loaded into the systemcontroller in use.

FIG. 6 shows a graphical representation of an alternate data set for usewith the breathing assistance system of FIG. 2, 3 or 4, the alternativedata compared to or used alongside the equivalent data from the tableshown graphically in FIG. 5, the graph lines showing curvesrepresentative of two different steady flow rates for a range of ambientatmospheric temperatures with little movement of the ambient air, and arange of target temperatures for a given flow and ambient temperature,and the same steady flow rates shown over a range of ambienttemperatures with high convective heat loss from the humidificationchamber, the data from the look-up table loaded into the systemcontroller in use.

FIG. 7 shows a schematic representation of some of the connectionsbetween a controller suitable for use with the breathing assistancesystem of FIG. 2, 3 or 4, and other components of the preferred form ofbreathing assistance system as shown in FIG. 2, 3, or 4.

FIG. 8a shows a graph of measured experimental data of flow, dew point,chamber exit or chamber outlet temperature under conditions of highambient temperature using a breathing assistance system such as thatshown in FIG. 2, 3 or 4.

FIG. 8b shows a similar graph to FIG. 8a , for conditions of low ambienttemperature.

FIG. 9 shows a schematic representation of part of the programming for acontrol system that is used by the breathing assistance system of FIG. 2or FIG. 3 to adjust the flow rate through the system so that it remainssubstantially constant when the geometry of the system changes, thecontrol mechanism containing two P.I.D. control filters, one for largedeviation and one for small deviation from the set flow rate, thecontrol mechanism also containing an averaging filter located in afeedback path to compare the measured flow with the set flow rate.

FIG. 10a shows a schematic representation of part of the programming fora control system that is used by the breathing assistance system of FIG.2 or FIG. 3 so that average and intra breath flow can be controlled witha low-pass filter also incorporated as part of the programming and usedfor determining whether coarse or fine flow control is used.

FIG. 10b shows a schematic representation of part of the programming fora control system which incorporates dual feedback loops for improvedflow control in the system of FIG. 2 or FIG. 3, the dual feedback loopsallowing separate control filters, so that average and intra breath flowcan be controlled.

FIG. 11 shows a schematic diagram of the system of FIG. 10b , with theaddition of a further feedback path from the flow generator to thecompensation filter, to assist in compensating for the non-linear natureof the breathing system shown in FIGS. 2, 3 and 4.

FIG. 12 shows a graph of motor speed for a number of example interfaces,demonstrating that humidity can be controlled to an appropriate levelfor either a mask or a nasal cannula (which require different motorspeeds, with the system remaining stable and producing an appropriatehumidity level at both high-speed and at low-speed).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A schematic view of a user 2 receiving air from a modular assistedbreathing unit and humidifier system 1 according to a first variant orembodiment of the invention is shown in FIGS. 2a and 2b . The system 1provides a pressurised stream of heated, humidified gases to the user 2for therapeutic purposes (e.g. to reduce the incidence of obstructivesleep apnea, to provide CPAP therapy, to provide humidification fortherapeutic purposes, or similar) The system 1 is described in detailbelow.

The assisted breathing unit or blower unit 3 has an internal compressorunit, flow generator or fan unit 13—generally this could be referred toas a flow control mechanism. Air from atmosphere enters the housing ofthe blower unit 3 via an atmospheric inlet 40, and is drawn through thefan unit 13. The output of the fan unit 13 is adjustable—the fan speedis variable. The pressurised gases stream exits the fan unit 13 and theblower unit 3 and travels via a connection conduit 4 to a humidifierchamber 5, entering the humidifier chamber 5 via an entry port or inletport 23. The humidifier chamber 5 in use contains a volume of water 20.In the preferred embodiment, in use the humidifier chamber 5 is locatedon top of a humidifier base unit 21 which has a heater plate 12. Theheater plate 12 is powered to heat the base of the chamber 5 and thusheat the contents of the chamber 5. As the water in the chamber 5 isheated it evaporates, and the gases within the humidifier chamber 5(above the surface of the water 20) become heated and humidified. Thegases stream entering the humidifier chamber 5 via inlet port 23 passesover the heated water (or through these heated, humidifiedgases—applicable for large chamber and flow rates) and becomes heatedand humidified as it does so. The gases stream then exits the humidifierchamber 5 via an exit port or outlet port 9 and enters a deliveryconduit 6. When a ‘humidifier unit’ is referred to in this specificationwith reference to the invention, this should be taken to mean at leastthe chamber 5, and if appropriate, the base unit 21 and heater plate 12.The heated, humidified gases pass along the length of the deliveryconduit 6 and are provided to the patient or user 2 via a user interface7. The conduit 6 may be heated via a heater wire (not shown) or similarto help prevent rain-out. The user interface 7 shown in FIG. 2a is anasal mask which surrounds and covers the nose of the user 2. However,it should be noted that a nasal cannula (as shown in FIG. 2b ), fullface mask, tracheostomy fitting, or any other suitable user interfacecould be substituted for the nasal mask shown. A central controller orcontrol system 8 is located in either the blower casing (controller 8 a)or the humidifier base unit (controller 8 b). In modular systems of thistype, it is preferred that a separate blower controller 8 a andhumidifier controller 8 b are used, and it is most preferred that thecontrollers 8 a, 8 b are connected (e.g. by cables or similar) so theycan communicate with one another in use. The control system 8 receivesuser input signals via user controls 11 located on either the humidifierbase unit 21, or on the blower unit 3, or both. In the preferredembodiments the controller 8 also receives input from sensors located atvarious points throughout the system 1. FIG. 7 shows a schematicrepresentation of some of the inputs and outputs to and from thecontroller 8. It should be noted that not all the possible connectionsand inputs and outputs are shown—FIG. 7 is representative of some of theconnections and is a representative example. The sensors and theirlocations will be described in more detail below. In response to theuser input from controls 11, and the signals received from the sensors,the control system 8 determines a control output which in the preferredembodiment sends signals to adjust the power to the humidifier chamberheater plate 12 and the speed of the fan 13. The programming whichdetermines how the controller determines the control output will bedescribed in more detail below.

A schematic view of the user 2 receiving air from an integratedblower/humidifier system 100 according to a second form of the inventionis shown in FIG. 3. The system operates in a very similar manner to themodular system 1 shown in FIG. 2 and described above, except that thehumidifier chamber 105 has been integrated with the blower unit 103 toform an integrated unit 110. A pressurised gases stream is provided byfan unit 113 located inside the casing of the integrated unit 110. Thewater 120 in the humidifier chamber 105 is heated by heater plate 112(which is an integral part of the structure of the blower unit 103 inthis embodiment). Air enters the humidifier chamber 105 via an entryport 123, and exits the humidifier chamber 105 via exit port 109. Thegases stream is provided to the user 2 via a delivery conduit 106 and aninterface 107. The controller 108 is contained within the outer shell ofthe integrated unit 100. User controls 111 are located on the outersurface of the unit 100.

A schematic view of the user 2 receiving air from a further form ofbreathing assistance system 200 is shown in FIG. 4. The system 200 canbe generally characterised as a remote source system, and receives airfrom a remote source via a wall inlet 1000. The wall inlet 1000 isconnected via an inlet conduit 201 to a control unit 202, which receivesthe gases from the inlet 1000. The control unit 202 has sensors 250,260, 280, 290 which measure the humidity, temperature and pressure andflow respectively of the incoming gases stream. The gases flow is thenprovided to a humidifier chamber 205, with the gases stream heated andhumidified and provided to a user in a similar manner to that outlinedabove. It should be noted that when ‘humidifier unit’ is referred to fora remote source system such as the system 200, this should be taken tomean as incorporating the control unit 202—the gases from the remotesource can either be connected directly to an inlet, or via the controlunit 202 (in order to reduce pressure or similar), but the control unitand the humidifier chamber should be interpreted as belonging to anoverall ‘humidifier unit’. If required, the system 200 can provide O₂ oran O₂ fraction to the user, by having the central source as an O₂source, or by blending atmospheric air with incoming O₂ from the centralsource via a venturi 90 or similar located in the control unit 202. Itis preferred that the control unit 202 also has a valve or a similarmechanism to act as a flow control mechanism to adjust the flow rate ofgases through the system 200.

Sensors

The modular and integrated systems 1, 100 and 200 shown in FIGS. 2, 3and 4 have sensors located at points throughout the system. These willbe described below in relation to the breathing assistance system 1.

The preferred form of modular system 1 as shown in FIG. 2 has at leastthe following sensors in the following preferred locations:

1) An ambient temperature sensor 60 located within, near, or on theblower casing, configured or adapted to measure the temperature of theincoming air from atmosphere. It is most preferred that temperaturesensor 60 is located in the gases stream after (downstream of) the fanunit 13, and as close to the inlet or entry to the humidifier chamber aspossible.

2) A humidifier unit exit port temperature sensor 63 located either atthe chamber exit port 9, or located at the apparatus end (opposite tothe patient end) of the delivery conduit 6. Outlet temperature sensor 63is configured or adapted to measure the temperature of the gases streamas it exits chamber 5 (in either configuration the exit port temperaturesensor 63 can be considered to be proximal to the chamber exit port 9).

Similarly, sensors are arranged in substantially the same locations inthe integrated system 100 shown in FIG. 3 and the system 200 of FIG. 4.For example, for the integrated system of FIG. 3, an ambient temperaturesensor 160 is located within the blower casing in the gases stream, justbefore (upstream of) the humidifier chamber entry port 123. A chamberexit port temperature sensor 163 is located either at the chamber exitport 109 and is configured to measure the temperature of the gasesstream as it exits chamber 105 (in either configuration the exit porttemperature sensor 163 can be considered to be proximal to the chamberexit port 109). Alternatively, this sensor can be located at theapparatus end (opposite to the patient end) of the delivery conduit 106,for either embodiment. A similar numbering system is used for thebreathing assistance system shown in FIG. 4—ambient temperature sensor260, fan unit 213, chamber exit port temperature sensor 263 located atthe chamber exit port 209, etc.

It is also preferred that the breathing assistance system 1 (and 100,200) also has a heater plate temperature sensor 62 located adjacent tothe heater plate 12, configured to measure the temperature of the heaterplate. The breathing assistance system(s) having a heater platetemperature sensor is preferred as it gives an immediate indication ofthe state of the heater plate. However, it is not absolutely necessaryto for the system(s) to have the heater plate temperature sensor inorder to reduce the invention to practice.

It is also most preferred that the systems also have a flow probe—flowprobe 61 in system 1—located upstream of the fan unit 13 and configuredto measure the gases flow. The preferred location for the flow probe isupstream of the fan unit, although the flow probe can be locateddownstream of the fan, or anywhere else appropriate. Again, it ispreferred that a flow probe forms part of the system, but it is notabsolutely necessary for a flow probe to be part of the system to reducethe invention to practice.

The layout and operation of the breathing assistance system 1 will nowbe described below in detail. The operation and layout of the systems100 and 200 is substantially the same, and will not be described indetail except where necessary.

For the breathing assistance system 1, the readings from all of thesensors are fed back to the control system 8. The control system 8 alsoreceives input from the user controls 11.

Further alternative additional sensors and their layout will bedescribed in more detail later.

Humidity Control Method

In the most preferred embodiment, the control system 8 has at least onedata set pre-loaded into the controller. The data that forms the dataset is pre-measured or pre-calculated under controlled conditions (e.g.in a test area or laboratory) for a specific system configuration withspecific components (e.g. system 1 or system 100, or system 200, with aparticular, specific blower unit and humidifier unit used to gather thedata). The data is gathered under a number of condition ranges that willtypically be encountered in use, with the pre-measured (pre-set) datathen being loaded as integral software or hardware into the controller 8for the production systems, or as data to be used in e.g. a fuzzy logicalgorithm for humidity control.

A data set particularly suitable for use with system 1 is shown as agraph in FIG. 5. The X-axis shows a range of ambient temperatures, from18° C. to 35° C. In use, the ambient temperature of the gases in thebreathing assistance system before or upstream of the chamber 5 ismeasured by the ambient temperature sensor 60, and the ambienttemperature data is relayed to the controller 8. It is most preferredthat the temperature sensor 60 measures the ambient temperature of thegases just before the gases enter the chamber 5. In order to create thedata set, a typical system 1 is placed in an environment where theambient temperature can be kept at a known, constant level over a rangeof temperatures.

In the preferred form in use, a user chooses a flow rate by adjustingthe controls 11. The controller 8 receives the input from the usercontrols 11 and adjusts the fan speed to substantially match thisrequested flow rate (either by altering the speed of the fan to a speedthat is known to substantially correspond to the required flow for theparticular breathing circuit configuration, or by measuring the flowusing flow probe 61 and using a feedback mechanism via controller 8 toadjust the flow rate to the level required or requested). Sevendifferent constant flow rates are shown in the graph of FIG. 5, forseven different constant fan speeds. The lines 70-76 correspond todifferent flow rates as follows: Line 70—a flow rate 15 litres/minute.Line 71—a flow rate of 20 litres/minute. Line 72—a flow rate of 25litres/minute. Line 73—a flow rate of 30 litres/minute. Line 74—a flowrate of 35 litres/minute. Line 75—a flow rate of 40 litres/minute. Line76—a flow rate of 45 litres/minute.

The Y-axis shows a range of target chamber temperatures. That is, forany given fan speed (flow rate and pressure), and any given ambienttemperature, there is a ‘best’, or ‘ideal’ target outlet temperature forthe gases in the chamber 5 above the water 20—the target outlettemperature as shown on the Y-axis. This ‘ideal’ temperature is the dewpoint temperature for a given constant flow and constant ambienttemperature. That is, the temperature at which the gases can exit thechamber 5 at the required saturation (required level of humidity) andthen be delivered to the user 2 at the correct temperature and pressurefor effective therapy. As the gases exit the chamber 5, the gasestemperature is measured by the chamber exit port temperature sensor 63.The controller 8 is adapted to receive the temperature data measured bythe chamber exit temperature sensor 63 and the data relating to thetemperature of the gases entering the chamber 5 (as measured by ambienttemperature sensor 60). The flow rate has been previously set to aconstant value, as outlined above, so the controller 8 already ‘knows’the constant flow rate. As the controller 8 ‘knows’ both the flow rateand the ambient temperature, it can, for example, look up the ‘ideal’target outlet temperature from the range incorporated into thepre-loaded data set (e.g. the data shown graphically in FIG. 5). Thecontroller 8 then compares the measured value of chamber exittemperature to the ‘ideal’ target chamber temperature for the given,known flow rate and ambient temperature. If the measured value of targettemperature does not match the ‘ideal’ target value, the controller 8generates or determines a suitable control output, and adjusts the powerto the heater plate accordingly, either increasing the power to increasethe temperature of the gases within the chamber 5, or decreasing thepower to decrease the gases temperature. The controller 8 adjusts thepower in this manner in order to match the temperature measured at theoutlet or exit port with the required target temperature. In thepreferred embodiment, the mechanism by which the controller 8 adjuststhe output characteristics is via a Proportional-Integral-Derivativecontroller (P.I.D. controller) or any one of a number of similarmechanisms which are known in the art.

The controller could also generate or determine a suitable controloutput by, for example, using a fuzzy logic control algorithm loadedinto the controller 8, or mathematical formulae which utilise themeasured temperature and flow data as variables in the equations.

Examples of mathematical formulae are shown below. These correspondgenerally to the data shown graphically in FIG. 5, for the range of flowrates from 15 to 45 litres/min.

45 T_(cs) = −0.0005 T_(amb) ⁴ + 0.055 T_(amb) ³ − 2.1234 T_(amb) ² +35.785 T_(amb) − 186.31 40 T_(cs) = −0.0005 T_(amb) ⁴ + 0.0578 T_(amb) ³− 2.2311 T_(amb) ² + 37.554 T_(amb) − 196.98 35 T_(cs) = −0.0006 T_(amb)⁴ + 0.0625 T_(amb) ³ − 2.4283 T_(amb) ² + 41.178 T_(amb) − 221.29 30T_(cs) = −0.0006 T_(amb) ⁴ + 0.0669 T_(amb) ³ − 2.6156 T_(amb) ² +44.613 T_(amb) − 244.25 25 T_(cs) = −0.0006 T_(amb) ⁴ + 0.0696 T_(amb) ³− 2.7315 T_(amb) ² + 46.76 T_(amb) − 258.75 20 T_(cs) = −0.0007 T_(amb)⁴ + 0.0736 T_(amb) ³ − 2.8942 T_(amb) ² + 49.651 T_(amb) − 277.53 15T_(cs) = −0.0007 T_(amb) ⁴ + 0.0776 T_(amb) ³ − 3.0612 T_(amb) ² +52.611 T_(amb) − 296.71

Example

The therapy regime of a user 2 specifies a certain flow rate andpressure, for example a flow of 45 litres/min. The speed of the bloweror fan unit 13 is set (via the controls 11) to deliver gases at thisflow rate. If a flow probe 61 is part of the system, this flow rate canbe dynamically adjusted by feeding back a real-time flow reading fromthe flow sensor or flow probe 61 to the controller 8, with thecontroller 8 adjusting the fan speed as necessary. This can be done viaa P.I.D. controller that comprises part of the controls 8 as describedin detail below, or similar. It is preferred that the flow rate isdynamically adjusted and monitored. However, if a flow probe is not partof the system, then the flow rate is assumed or calculated from the fanspeed, and is assumed to be constant for a constant fan power level. Theflow rate of 45 litres/minute is shown by line 76 on the graph of FIG.5. In this example, the user 2 is sleeping in a bedroom having anambient temperature of substantially 30° C. Air at 30° C. enters thebreathing assistance apparatus and as it passes through the fan andconnecting passages within the casing, it warms up slightly. Thetemperature of the air just before it enters the humidifier chamber ismeasured by the ambient temperature sensor 60. As the ambienttemperature and the flow rate are known, the controller 8 can calculatethe required target temperature, as shown on the Y-axis of the graph ofFIG. 5. For this particular example, it can be seen that the chambertarget temperature is 42° C. The chamber exit temperature sensor 63measures the temperature of the gases as they exit the chamber 5 (thegases temperature at the exit point will be substantially the sametemperature as the gases in the space above the chamber contents 20). Ifthe gases temperature as measured by the chamber exit temperature sensor63 is not 42° C., then the controller 8 determines and generates asuitable control output which alters the power to the heater plate 12accordingly. As above, if the ambient temperature as measured by theambient temperature sensor 60 changes, this can be fed back to thecontroller 8 and the outputs altered as appropriate using a P.I.D.control algorithm or similar.

One of the advantages of this system over the systems disclosed in theprior art is as follows: in prior art systems, as the ambienttemperatures approach the target dew point temperature, the heater platewill draw less power and not raise the temperature of the water in thehumidifier chamber as much. Therefore the gases will tend not be fullysaturated as they exit the chamber. The method outlined above overcomesthis problem by using values of ambient temperature or more preferablychamber inlet temperature, chamber exit temperature and flow rate for asystem of a known configuration, in order to produce a target chamberexit temperature which is considered to be substantially the best or‘ideal’ temperature for gases saturation and delivery to a user for aset flow rate and a particular ambient temperature.

Another advantage is that the system 1 can accurately control thehumidity level without the need for an accurate humidity sensor.

Another advantage is that when gas is delivered to the humidifierchamber from the compressor or blower, and this incoming gas has anincreased temperature, the chamber temperature can be accuratelycompensated to achieve the desired dew point. This is particularlyadvantageous if the air or gases entering the chamber are warm, and alsoin situations when the temperature increases as the flow increases. Inoperation, any flow generator causes an increase in air temperaturebetween the inlet from atmosphere and the outlet. This change intemperature can be more pronounced in some types of flow generator. Thetemperature of components of the system can change between the time atwhich the system is first activated and some time afterwards (e.g. overa reasonably prolonged period of time such as 1-2 hours). That is,components of the system can heat up as the system is operating, withthe system taking some time to reach a steady state of operation. Ifthese components are located in or adjacent to the air path between thepoint at which air enters the system, and the point at which the airenters the chamber, then the temperature of these gases is going tochange—there is going to be some heat transfer from these components tothe gases as the gases travel along this path. It can therefore be seenthat measuring the temperature of the gases as they enter the chamberreduces the likelihood of introducing a temperature measurement errorinto the control calculations, as the temperature of the gases at thepoint of entry to the system when the system has reaches a steady stateof operation may be different from the temperature of the gases at thepoint of entry to the chamber. However, it has generally been found thatalthough it is most preferable to measure gases temperature at the pointof entry to the chamber, it is also acceptable in most circumstances tomeasure atmospheric gases temperature.

The method described above is substantially similar for the integratedapparatus 100, or the apparatus 200, although the pre-set orpre-measured and pre-loaded values in the look-up table may differ asthe apparatus has a slightly different configuration. In other forms,the user could choose a pressure rate (and the data set would bemodified for pressure values rather than flow values).

The apparatus and method described above has been found to provideimproved control of the gases characteristics at the point of deliveryto the user 2, over systems and methods known in the prior art. Thesystem and method described above goes some way towards overcoming theproblems with prior art methods and apparatus. The system and methoddescribed above controls the output characteristics with the aim ofproducing gases at the chamber exit which are fully saturated—that is,the gases exiting the chamber are at dew point or very close to dewpoint for a given temperature. The system output characteristics arevaried for the target dew point temperature, rather than the chamberexit temperature.

If the system has a user display, the dew point (or alternatively, theabsolute humidity, or both dew point and absolute humidity) can bedisplayed rather than the chamber outlet temperature. As outlined above,the chamber outlet temperature can be an inaccurate indication of thehumidity level of the gases exiting the humidifier chamber. This hasbeen experimentally verified with a modular system substantially similarto that of FIG. 2. Data was measured over a full range of flow rates,from approximately 15 litres/minute to approximately 45 litres/minute.The chamber outlet temperature and the dew point at the chamber outletformed part of the measured data. The data was measured for onesubstantially constant ambient temperature (although this was alsomeasured throughout the test to remove uncertainty). The data collectedis shown graphically in FIG. 8a and FIG. 8b , which show flow rate onthe Y-axis against time on the X-axis. In the graph of FIG. 8a , thedata was gathered for conditions of high ambient temperature. Themeasured flow rates are shown on the graph by the points 801. Line 802shows the ambient temperature. Line 803 shows the measured chamberoutlet temperature. Line 804 a shows the measured dew point (Tdmeasured). Line 805 a shows the displayed dew point (Td displayed). Ascan be seen, the ambient temperature remains substantially the same(increasing slightly with time). The chamber outlet temperature changesfrom 39° C. to 41° C. The actual measured outlet dew point fluctuatesaround a substantially constant level. However, these fluctuations orvariations mostly occur during flow transitions. The displayed dew pointremains constant for the entire flow range.

FIG. 8b shows a similar graph to FIG. 8a , but for conditions of lowambient temperature (that is, 18-20° C.) and flow rates over the range45-15 Litres/min. The chamber outlet temperature is not displayed as itis very close to dew point. It should be noted that in the preferredform, dew point is displayed when the temperature reaches 30° C. only.Patients should not use the humidifier when humidity is too low. It canbe seen that ambient temperature oscillations have caused transientbehaviour to appear in the measured dew point. However, despite this,the displayed dew point (Td displayed) as shown by line 805 b can beseen to be ‘tracking’ the actual dew point (Td measured as shown by line804 b) very consistently. It should be noted that at 12 minutes, theflow rate was turned briefly from 45 Litres/min to 15 Litres/min,causing a small overshoot, as can be seen on the graph of FIG. 8b . Athigh flows of 45-40 Litres/min, the heater plate could not maintain thetargeted temperature and the humidity output was lower than Td 37° C.This is reflected in the displayed dew point.

Further preferred variations and embodiments will now be described,which add to the improved control of the gases characteristics.

Further Alternative Sensor Layouts

In a variant of the apparatus and method outlined above, the system(system 1 or system 100 or system 200) also has additional sensors asoutlined below.

1) A patient end temperature sensor 15 (or 115 or 215) is located at thepatient end of the delivery conduit 6 (or alternatively in or on theinterface 7). That is, at or close to the patient or point of delivery.When read in this specification, ‘patient end’ or ‘user end’ should betaken to mean either close to the user end of the delivery conduit (e.g.delivery conduit 6), or in or on the patient interface 7. This appliesunless a specific location is otherwise stated. In either configuration,patient end temperature sensor 15 can be considered to be at or close tothe user or patient 2. The reading from the patient end temperaturesensor 15 is fed back to the controller 8 and is used to ensure that thetemperature of the gases at the point of delivery substantially matchesthe target patient temperature of the gases at the chamber exit (thetarget patient temperature is the target dew point temperature at thechamber exit). If the reading from the patient end temperature sensor 15indicates that the gases temperature is dropping as it travels thelength of the delivery conduit 6, then the controller 8 can increase thepower to the conduit heater wire (shown as wire 75 on FIG. 2a —not shownbut present in the alternative preferred forms of breathing assistancesystem 200 and 400 shown in FIGS. 3 and 4, and the system shown in FIG.2b ) to maintain the gases temperature. If the power available to theconduit heater wire 75 is not capable of allowing the gases at the pointof delivery to equal the dew point temperature at the chamber exit 9then the controller 8 lowers the target chamber exit temperature (tolower the dew point temperature). The controller 8 lowers the chamberexit temperature to a level at or close to the maximum gases temperaturethe conduit heater wire is able to deliver to the patient as measured bythe patient end temperature sensor 15. The controller 8 is loaded with apredetermined data set, and adjusts the power to the heater plate, orthe conduit heater wire, or both, by using this data (which is similarto that shown in graphical form in FIG. 5). For a constant flow leveland for a measured ambient temperature as measured by ambienttemperature sensor 60 (which may change), there is an ideal patient endtemperature. The controller 8 adjusts the power output or outputs of theheater plate and the conduit to match the temperature at the patient endof the conduit (as measured by temperature sensor 15) with this idealtemperature.

The above method can be further refined for accuracy if other conditionsof the gases in the system are known—the gases conditions. For example,if the humidity level of the incoming gases to the blower is known, orthe gases pressure of the incoming gases. In order to achieve this,alternative embodiments of the systems 1, 100 and 200 described abovecan also have a gases condition sensor located in the incoming gas path(e.g. a humidity sensor or a pressure sensor). For the modular system 1,a humidity sensor 50 is shown located proximal to the atmospheric inlet40. For the integrated system 100, this is shown as humidity sensor 150(and so on). In a similar fashion to the control methods outlined above,the controller 8 is pre-loaded with a humidity level data set. For aconstant flow rate, and known ambient or external humidity level, thereis an ideal gases temperature at the chamber exit (or at the point ofdelivery to a user). The data set contains these ideal values for arange of ambient humidities and flow rates, similar to the values shownin graphical form in FIG. 5. The controller 8 adjusts the power outputof the heater plate, or the heater wire, or both, to match the measuredchamber exit temperature (or patient end temperature) with the ‘ideal’temperature retrieved from the data set in the memory of thecontroller). In a similar manner, the above method can be refined foraccuracy if the pressure level of the incoming gases to thehumidification chamber blower is known, locating a pressure sensor inthe incoming gas path to the humidification chamber (pressure sensor 80shown in the incoming gases path in FIG. 2 for the modular system.Pressure sensor 180 is shown in the incoming gases path in FIG. 3 forthe integrated system. Pressure sensor 280 is shown in the incominggases path in FIG. 4 for the central gases source system). It should benoted that if the data for the data set was plotted graphically forconditions of constant flow, ambient temperature and another gasescondition (e.g. humidity or pressure), the graphs would be required tobe plotted on three axes—X, Y and Z—the graphs would be‘three-dimensional’ when plotted.

A further variation on the layout or construction of the breathingassistance system is as outlined below:

It is intended in some embodiments that the gases exit the chamber at41° C. As the gases pass along the main delivery tube or conduit towardsthe interface, they are heated from 41° C. at the chamber exit to 44° C.at the end of the main delivery hose 6. At the end of the main deliveryhose the gases enters a smaller secondary, unheated delivery hose—e.g. 6a as shown on FIG. 2b . As they pass through the secondary hose 6 a thegases cool from a temperature of 44° C., to 37° C. as they enter theuser interface 7. 37° C. is considered to be the optimum deliverytemperature for the patient.

A further refinement of the method outlined above, with or without theadditional sensors, will now be described.

Compensating for Convective Heat Loss and Heat Gain of Flow Generators

As outlined in the prior art section, one problem that is known in theart is that of accurately controlling the output characteristics of asystem when there are a large number of variables which can affect theoutput characteristics. It has been found that one of the variables thathas an effect on the gases output characteristics is convective heatloss from the humidifier chamber 5. This convection can be caused bynatural factors such as temperature gradients in the room—“natural orfree convection” or by forced movement of air—“forced convection”.Forced convection could for example be caused by a ventilator or an airconditioner. Convection cooling of the humidifier chamber cansubstantially affect the dew point temperature at the humidifier chamberoutlet. A flow of air over the outside surfaces of the humidifierchamber—e.g. chamber 5 of system 1—will cause the temperature inside thechamber to drop. In order to compensate for this, more power is requiredat the heater plate to increase the temperature of the contents of thechamber 5. The output temperature at the chamber outlet is measured byoutlet temperature sensor 63, and the temperature loss will be ‘seen’ bythe controller 8 as it records a drop in temperature at the chamberoutlet. The controller 8 will increase the power to the heater plate 12to compensate for this (with a corresponding increase in heater platetemperature measured by the heater plate temperature sensor 62). Theeffect of this increase in power is to increase the heat transfer ratiofrom water to gas and the partial water pressure of gas inside thechamber, and consequently there is an increase in the dew pointtemperature.

Evaporation of non-boiling water is governed by Low Mass Transfer RateTheory and mass (water) transfer directly related to heat transfer. Sothe evaporation depends on the temperature of the incoming gas (and lessso on its humidity), temperature of water, flow and pressure. Flowdetermines not only flow of gas over water but also the movement ofwater. For example, stirring (forced convection) of water will increasethe evaporation. The evaporation rate is higher during a transition modeof a heater plate controller. The transition mode is characterized bylarger oscillations of temperature in the heater plate and likely causesan increased turbulence (free convection) in water by raising theNusselt number and its mass transfer analogue the Sherwood number. Thisis more noticeable at high ambient temperature, or more particularlyunder conditions where gases entering the humidifier are at a hightemperature, and when chamber outlet gas temperature is significantlyhigher than dew point. Convective heat loss causes dew point to increaseclose to the temperature of the gas.

Elevated chamber outlet temperature over dew point causes instability inthe control system. Any fluctuations of flow or convective heat losswill cause a quick increase in mass (water) transfer and subsequentlyhumidity of the gas. This instability is illustrated in FIG. 8a wheremeasured dew point (804) of the air at high ambient temperature cycleswhile measured temperature at the chamber outlet (803) remainsrelatively stable.

This is a typical problem of humidity output control in respiratorysupport devices that have incorporated both a flow generator andhumidifier (such as CPAP blower, BiPAP or non-invasive ventilatorsetc—see FIGS. 1, 2 and 3 for example) and have a typically targeted dewpoint at 31-32 degrees, rather than dew point close to a bodytemperature of 37° C. (high humidity with dew point 37° C. is typicallyused in high flow therapy and invasive ventilation). The flow generatorwill cause the temperature at the chamber inlet to be increased aboveusual ambient temperature (22-24° C.) by several degrees. The inlettemperature may become very close to or even exceed 31-32° C. Increasedambient (atmospheric) temperature significantly aggravates the problem.The increased chamber inlet temperature requires air to be heated toapproximately 36-41° C. or even higher (depending on the flow rate) toachieve a dew point of 31-32° C. The patients' physiological breathingor mechanical ventilation may also affect flow in the humidificationchamber and as a result exposure time of air in the chamber. All theseconditions combine to produce variable humidity output at the chamberoutlet. If the humidification chamber is exposed to environment, whichis usually the case for practicality, the convection heat loss can alsosignificantly alter the humidity output.

The convective heat loss (‘draft’) is created by airflows over andaround the ventilation equipment, and particularly the humidifierchamber. This can be particularly significant in designs where thechamber is at least partially exposed, particularly in ventilatedspaces. The flow velocities of the air vary in magnitude, direction andfluctuation frequency. Mean air velocities from below 0.05 m/s up to 0.6m/s, turbulence intensities from less than 10% up to 70%, and frequencyof velocity fluctuations as high as 2 Hz that contribute up to 90% ofthe measured standard deviations of fluctuating velocity have beenidentified in the occupied zone of rooms.

The convective heat loss can also be estimated by measuring flowintensity- or turbulence intensity (or both) over the chamber. This canbe achieved using thermal, laser or sonic anemometry, with sensorsmounted on the equipment (e.g. the humidifier base unit 21) so as tomeasure the flow or turbulence intensity at or close to the humidifierchamber 5.

For precision humidity control, compensation for convective heat loss isdesirable. This compensation is made easier if the controller 8 has theadvantage of a ‘convection compensation’ data set or sets to rely on, orif the controller has the advantage of an alternative ‘convectioncompensation’ method. The controller could be programmed with afuzzy-logic type rule-based system.

The data set shown graphically in FIG. 5 is calculated under conditionswhere there is little to no convective heat loss. This data is suitablefor use under conditions where there is low movement of the ambient air.In alternative forms, or variants of the apparatus and method outlinedabove, the controller 8 will switch to using alternative data as inputwhen the convective heat loss reaches a certain level—for example, ifthe controller 8 notes a large step change in the heater platetemperature as measured by the heater plate temperature sensor 62. Forexample, the data will be used as input for a fuzzy logic controlalgorithm, a mathematical formula or formulae, or similar.

FIG. 6 shows part of the data for use if or when ambient conditionschange during use to a ‘high convection’ condition—if during use thereis a flow of air over the apparatus, and in particular the humidifierchamber, and as a consequence there is a change from a low convectiveheat loss condition to a high convective heat loss condition. Thealternate data of FIG. 6 is created in the same way as the table shownin FIG. 5, but the pre-measured and pre-loaded conditions (flow andambient temperature) are for a system where at least the chamber 5 (or105 or 205) is experiencing a high level of convective heat loss. Thetarget temperature changes accordingly. In FIG. 6, part of thealternative data for use in a ‘high convective heat loss’ condition isshown. Two curves 501 and 502 are shown, representative of a steady flowrate of 15 litres/minute (501) and 45 litres/minute (502). A range ofambient temperatures (X-axis) and a range of target chamber exittemperatures for a given steady flow rate and ambient temperature areshown (Y-axis), in a similar manner to the data shown in FIG. 5. For thepurposes of comparison, the two equivalent steady flow lines (15litres/minute and 45 litres/minute) from FIG. 5 are also shown on thegraph as lines 503 (15 litres/minute) and 504 (45 litres/minute). It canbe seen that when the apparatus is subject to a ‘high-flow’ condition,the target chamber outlet temperature as shown on the Y-axis is lowerthan when the apparatus is subject to a ‘low-draft’ or low level ofconvective heat loss condition.

Similarly, alternate rule sets can be calculated and pre-loaded into thecontroller 8. The controller can switch between alternate fuzzy logicrule-sets depending on the ambient conditions as measured or assessed bythe method(s) outlined above—for example when the convective heat lossreaches a certain level assessed by the controller 8 noting a large stepchange in the heater plate temperature as measured by the heater platetemperature sensor 62.

In order for the controller 8 to assess whether it should be using datarepresentative of low convective heat loss or high convective heat loss,an assessment of the heat loss is required. In the preferred embodiment,this is calculated from the power required at the heater plate 12 tomaintain the correct chamber exit temperature. The controller 8 ispre-loaded with data values of heater plate power for known ambienttemperatures and flow rates (alternatively the controller utilises fuzzylogic rule sets). The controller 8 assesses whether the humidifierchamber is operating in a condition of high convective heat loss, or acondition of low convective heat loss, and adjusts or alters it'scontrol output accordingly (e.g. by utilising the fuzzy logic rule setsto change operating condition). The condition of ‘highest convectiveheat loss’ is defined as the condition (fast moving air) when thecontrolled chamber outlet temperature is close to dew point and furthercooling of the chamber does not increase the humidity. Tow convectiveheat loss' is defined as the condition (still air) when the controlledchamber outlet temperature is raised above the dew point temperature.This is explained further below:

Normally the controller 8 uses an algorithm or rule set of ‘lowconvective heat loss’ (still air, or low convective heat loss). When thechamber 5 is cooled from outside by convection (‘high convective heatloss’) the humidity output will increase. The target chamber outlettemperature for the method outlined above (i.e. using the data shown inFIG. 5) uses look up table data (or a rule set) that corresponds to aheater plate temperature range and/or duty cycle of the heater plate.The controller 8 will switch to data representative of ‘high convectiveheat loss’ if a target value of the chamber gas outlet temperature isreached and the corresponding heater plate temperature is higher than aset limit for a given time period (this change could also beincorporated as one of the rules in a fuzzy logic rule set). It shouldbe noted that if a system is used that does not have a heater platetemperature sensor, the heater plate power duty cycle can be usedinstead of the heater plate temperature to calculate the switchoverpoint—that is, if a target chamber gas outlet temperature is reached andthe power drawn by said heater plate is higher than a set value for agiven time period.

The controller 8 will decrease the target chamber gas outlet temperatureby an appropriate value.

Example

In the preferred embodiment, for the system 100 of FIG. 3, if a targetvalue of 39.5° C. of the chamber gas outlet temperature is reached andthe corresponding heater plate temperature (or calculated power) ishigher than 60-65° C. for five minutes, the controller 8 will determinea control output that decreases the target chamber gas outlettemperature by 0.25° C.

This new value also has a new corresponding heater plate temperatureand/or duty cycle (i.e. chamber gas outlet temperature 38.4° C. andheated plate temperature 87° C.). So, the targeted dew point temperatureis titrated until it has proper corresponding heated plate temperature(by a fuzzy logic algorithm in the controller 8). If the heater platetemperature is significantly higher than the corresponding chamber gasoutlet temperature then the new targeted value is approached quicker.For example, if the heater plate temperature is more than 10° C. higherthen the new targeted value is reached in less time (i.e. 0.5° C. lower)etc. This drop of the targeted chamber gas outlet temperature may varyaccording to flow and/or ambient/gas chamber inlet temperature. Forexample, at a flow rate of 45 Litres/minute and an ambient temperatureof 23° C. this drop can be of 0.1° C. for every 5° C. of heater platetemperature. At an ambient temperature 30° C. it can be 0.7° C. forevery 5° C. of the heater plate temperature. Moreover, the drop of thetarget temperature can be non-linear.

In alternative embodiments, the heater plate temperature, the heaterplate duty cycle, the heater plate power, the duty cycle of the heatedtube, or the heated tube power can be used for estimation of theconvective heat losses. The heated tube has a larger surface area andwill therefore react quicker to convection changes.

The same principle as outlined above is applied in reverse when theconvective heat loss is decreasing after it has increased. Time limitsand steps of the chamber gas outlet temperature increase or decrease mayvary.

The displayed dew point can be corrected in a way that tracks actual dewpoint during the transition time.

In other alternate embodiments, multiple sets of data can be used fordifferent levels of convective heat loss, with the controller 8 usingone, some or all of the data sets to determine the control output fordifferent convective heat loss ranges, for example by using fuzzy logiccontrol algorithms, mathematical formulae or similar.

In yet another alternative embodiment, the use of multiple data sets canbe avoided by using a single data set, and modifying the target chamberoutlet temperatures as follows. If the flow rate, the ambienttemperature and the heater plate power use or heater plate temperatureare known, the target chamber outlet temperature can be modifiedaccording to the (known and changing) level of heater plate power. (ortemperature) for any given ambient temperature and flow rate. In thisway, the level of ‘draft’ or convective heat loss, for example, can becalculated from heater plate power used. The target chamber outlettemperature is modified to provide accurate dew point control for arange of convective heat loss conditions, by applying a correctionfactor or correction algorithm to the data in e.g. the data set used tocreate the graphs of FIG. 5. For example, if using heater plate power,the calculation can be made as follows: The required heater plate powerfor any given target chamber outlet temperature and flow rate for lowconvective heat loss conditions is known, and these values are stored inthe memory of the controller 8. In use, the controller 8 receives datarelating to the power used by the heater plate, and compares this to thestored data. If the measured data values and the stored data values arenot substantially similar (within +/−2% in the preferred form), thecontroller applies an inversely linear correction factor. For example,if the measured heater plate power is 10% greater than the stored values(indicative of a high convective heat loss condition), the controllerdecreases the target chamber outlet temperature by 10%.

It should be noted that heater plate temperature or any of the othermethods outlined above (e.g. heater plate temperature, conduit power,etc) could be used instead of the heater plate power as outlined in theexample above.

In a similar fashion, if one or more of the conditions of the gases isknown, then a correction algorithm or correction factor can be appliedto the (ambient condition) data stored in the memory of the controller8. The ambient conditions under which the data was measured and loadedare known (e.g. humidity and pressure). If the measured gases conditiondeviates from these base line conditions by a certain percentage (e.g.more than 2%), then the controller can apply a correction factor to thetarget chamber outlet temperature.

In the embodiments of a coupled blower and humidifier presentedschematically in FIGS. 1, 2 and 3, the chamber inlet temperature willusually be augmented with an increase of flow, or pressure, or both,from a flow generator. A fuzzy logic algorithm or algorithms can be usedto define the corrected chamber inlet temperature according to ambienttemperature or chamber entry/inlet temperature, and motor speed. Anincrease in the motor speed is usually accompanied by an increase of thechamber inlet temperature. Furthermore the known motor speed can be usedby the controller for defining humidity and temperature regimesaccording to a known interface attached at the patient end of thedelivery conduit. For example, the lower motor speed associated with amask interface (as opposed to a nasal cannula) can be used in thealgorithm to control humidity output from the system to an appropriatelevel for a mask. When a mask is used, a dew point of 31° C. isrequired. A small or large nasal cannula, or a tracheostomy fitting,require a dew point of 37° C. This is shown in FIG. 12, which shows agraph of motor speed for a number of example interfaces—a higher fan RPMis required for nasal cannula applications, and a lower fan RPM isrequired for mask applications. The RPM output of the motor can be keptmore stable by using the control method outlined above. The experimentalresults shown in FIG. 12 demonstrate that humidity can be controlled toan appropriate level for either a mask or a nasal cannula (which requiredifferent motor speeds, with the system remaining stable and producingan appropriate humidity level at both high-speed and at low-speed). Thex-axis shows time in use (in seconds). The y-axis shows motor speed(RPM). Line 1201 shows the motor speed for the system in use with asmall nasal cannula. Line 1202 shows the motor speed for the system inuse with a large nasal cannula. Line 1203 shows the motor speed for thesystem in use with a tracheostomy interface. Line 1204 shows the motorspeed for the system in use with a mask.

There are other potential ways in which the delayed ‘self heating’effect of the blower as it gradually warms up or heats up during use canbe compensated for.

Firstly, after a period of time of steady work (e.g. one hour, twohours, etc), the humidity control algorithm can switch from using thechamber outlet temperature as a variable, to using the heater platetemperature.

Secondly, a time component can be implemented in the control algorithm(e.g. after one hour of work the target chamber outlet temperature canbe increased by e.g. 0.5° C.

Thirdly, “the heat-up compensation factor” can be used. This factor canbe calculated using: time of work, duty cycle of heater, and heaterplate temperature. If the duty cycle or heater plate temperature changesover time, under conditions of steady flow rate and ambient temperature,then this indicates that the air coming from the blower is becominghotter with time, and this has to be compensated for.

Control for Constant Flow Rate

In the most preferred forms of the invention, the systems 1, 100 or 200also have a flow control system, which is adapted to control the flowthrough the system and keep this aligned as closely as possible to thedesired, user set, level. As outlined above, the flow and the humidityof the gases in the system are interlinked. As outlined above, in priorart systems, it is normal for the fan to be set to a constant speed, andit is assumed that the flow rate will remain substantially constant ifthe fan speed remains constant, or that the pressure at the point ofdelivery to the patient is constant. However, the flow can be affectedby changes in the system (which affects the humidity), even if the powerto the fan remains constant, or the fan speed remains constant. This isespecially true if the conduit, or interface, or both, have a relativelylow resistance to flow. The difference or deviation between themagnitude of the measured or actual flow against the magnitude of theuser-set flow can be characterised as a ‘large deviation’ or a ‘smalldeviation’. In the preferred embodiment, the difference between theactual flow rate and the desired (user-set) flow rate determines whetherthe controller 8 uses fine control or coarse control to match the actualflow rate to the desired flow rate.

For example, in the preferred form of system 1, when the system is firstturned on or activated, it ‘warms up’ prior to use. As it warms up, theflow rate approaches the user set point. A user will generally not bewearing their interface during the warm up period, and the interface maynot be connected to the delivery conduit. When a user puts theirinterface on, or connects the interface to the conduit, the flow ratewill decrease as the resistance to flow will increase. This can cause auser discomfort. Other unwanted side effects can also occur—for examplea change in the concentration of oxygen delivered, or a change in thedelivered humidity. The change in flow rate due to the increasedresistance to flow will be large or a large proportion or percentage ofthe overall flow rate, and can result in a large deviation of themeasured flow from the user set flow. Another example of a large flowdeviation would be for example if the user interface is changed orswapped e.g. from a full face mask to a nasal mask or a nasal cannula.There will be a change in the flow rate that may be characterised as alarge deviation from the user set flow—the difference between themeasured flow and the user set flow will be large. Large deviations canalso occur if e.g. small-bore nasal cannulas are swapped for large-borecannulas.

In contrast, there are changes to the flow rate through the system thatcan be characterised as ‘small deviations’. Some examples of changes tothe system which cause ‘small deviations’ from the user-set flow rateare as follows: If the geometry of the delivery conduit changes (e.g. ifa user turns over in their sleep and alters the way the delivery conduitis flexed or bent), then there will be a small relative or small changeor percentage change in the flow rate, and the deviation of the actualflow rate from the user set flow rate will also be small. Smalldeviations from the user set flow may also occur for example if theposition of the user interface on the user's face or in their nostrilschanges.

For the purposes of this specification, a base flow rate is set asfollows: by the user defining the ‘user set flow rate’. The flow ratethrough the system is measured, continuously or periodically giving the‘actual flow rate’ (e.g. via the flow probe 61). As long as the actualflow rate as measured matches the user set flow rate to within apredefined tolerance—e.g. 3 litres/minute, the controller 8characterises the flow rate as within tolerance—that is, there is not a‘large deviation’ between the actual measured flow rate and user setflow rate. If the measured flow rate is different from the user-set flowrate by more than the predefined tolerance of 3 litres/minute or morefrom the set base flow rate, the controller 8 characterises this as a‘large deviation’ in a similar manner to that outlined above. Incontrast, if there is a difference between the measured flow rate andthe user-set flow rate that is smaller than 3 litres/minute, this ischaracterised as a small deviation. It should also be noted that inalternative embodiments, the controller could work from a percentagedeviation from the user set flow rate, rather than an empirical changesuch as the 3 litres/minute of the preferred embodiment described above.

In the preferred embodiment, the control system or control algorithmloaded into the controller 8 is designed to switch between coarsecontrol and fine control, depending on whether there has been a largedeviation or a small deviation. If the controller ‘sees’ a largedeviation or a step change in the flow rate, it uses coarse controlparameters to restore the flow rate to the rate set by a user. If theflow rate is changing slowly, or if there is a small deviation in theflow rate, the controller 8 uses fine control parameters to adjust theflow rate.

To avoid system or measurement deviations associated with noise or witha patient breathing on the system triggering coarse control, the actualmeasured flow used is an average flow calculated over a period of timegreater than a few breath periods, rather than the instantaneouslymeasured flow.

A pre-loaded control system or systems (or a control algorithm oralgorithms, or fuzzy logic rule set) which is incorporated as part ofthe controller 8, and which acts on the system 1 (or 100, or 200) tosmooth the flow rate with the aim of delivering constant flow to a userundergoing humidification therapy is useful as it allows the flow to beset, and known. The flow is independent of the interface being used, thefit of the interface on a user, and the depth of the users breathing.This is particularly useful if a user is undergoing O₂ therapy forexample by using the system 200. If the flow of O₂ provided by e.g. acentral gases supply (provided to the humidifier chamber via a wallinlet and conduit) is known (measured by the flow probe), and the flowrate from a separate atmospheric supply is known (either measured by aseparate flow probe, or calculated from the system dimensions (e.g. theventuri dimensions) and the measured flow rate, using an algorithm inthe controller), then a look-up table loaded in the controller 208 cancalculate the O₂ fraction in the blended humidified air. For example,the difference in airflow between a cannula interface and a tracheainterface is typically 5 litres/minute or greater for the same user. Ifthe separate flow rates from atmosphere and the central supply areknown, the O₂ fraction can be set via user controls 11 to known valuesfor either of these interfaces without the need for an O₂ sensor. Also,by having a system that has a flow sensor which feeds back to thecontroller 208 and which sets the flow irrespective of the interface orbreathing pattern of the patient, the humidity can be preciselycontrolled as outlined herein. Therefore, with a preset flow thebreathing assistance system can deliver precise oxygen fractions andhumidity without the need for an oxygen sensor or humidity sensor.Precise flow control enables precise delivery of blended oxygen. Preciseflow control also enables precise control of the humidity levels in thegases (for example blended oxygen) delivered to the patient.

A schematic diagram showing the operation of a control system 300 isshown in FIG. 9. In the preferred form, the controller 8 (or 108 or 208)is loaded with a control system 300. The controller 8 uses P.I.D.control algorithms from a P.I.D. filter 313 as the coarse controlparameters or large deviation control parameters. In the filter 313, the‘P’ or proportional part is shown as 301, the ‘I’ or integral part isshown as 302, and the ‘D’ or derivative part is shown as 303. In thecontrol sub-system or algorithm, the fan unit 13 is shown, with the flowprobe 61 shown downstream of the fan unit 13. User input from thecontrols 11 is shown as arrow 304. A feedback signal 307 a is shown fromthe output of the control system or sub-system back to the front orinput end, to be fed into the filter 313 along with a signal indicativeof user set flow rate—user input 304 (it should be noted that when thephrase ‘user set flow rate’ is used in this specification, it can betaken to mean the user input signal 304). Arrow 311 shows the input intothe fan unit 13, which is the output or signal from the P.I.D. filter313 (either large deviation control filter 313 a or small deviationcontrol filter 313 b).

It can be seen from FIG. 9 that the filter 313 is divided into a ‘largedeviation control filter’ (313 a) and a ‘small deviation control filter’(313 b). The controller 8 switches between the two filters depending onthe parameters outlined above.

It should be noted that the coarse flow control or ‘large deviation’control can be achieved by using heater plate temperature, or tubetemperature, or both, as the input. If the temperature changes above acertain rate of change (a large deviation), then the controllerinitiates coarse control. The controller could also use the power orduty cycle of the heater plate or heater wire (or both), and the using alook-up table, formula or fuzzy logic algorithm. (this flow control canbe used as a stand alone or as a back-up control system). It may not beaccurate enough for oxygen therapy but can be potentially implemented insurgical humidification or high flow therapy (without O2).

Also data from the oxygen sensor (air enriched with O2) can be used asan input for fuzzy logic of flow control (change of O2% may reflect flowchange)

The flow control method and system described above can be furtherrefined to control the flow rate during the inspiration-expirationcycle, as described below.

Intra-Breath Control.

The flow control method described above addresses average flow—i.e. meanflow over a time period greater than that of a number of breathingcycles (e.g. three or more inspiration-expiration cycles). There is aneed for the implementation of a control system for maintaining constantflow over the course of a breath (inspiration/expiration). A preferredmanner in which this could be implemented is described below.

Flow through the conduit will vary as a patient inhales and exhales(i.e. over the course of a single breath or breathing cycle). Thepercentage amount by which the flow will vary over the course of abreath depends on a number of factors—for example the resistance of thetube/interface combination, the leak or seal around the cannula in theflares and the size of the breath taken. A very high resistance conduitand cannula combination is unlikely to need a control system formaintaining constant flow over the course of a breath. However, a lowresistance interface such as a nasal cannula for use with the system 1,100, or 200 is more likely to need a control system—the variation in theflow can be relatively large.

In some circumstances flow variation may actually be beneficial—it mayreduce the work required by a user to breathe, and may be morecomfortable for a user as the pressure at the nose during expiration islower than it would otherwise be for a constant flow device. In othercircumstances it may be beneficial to have a more constant flow throughthe tube. This will give a greater pressure during expiration and causehigher PEEP. This is useful and advantageous for treating somerespiratory ailments. For a relatively low resistance tube (and low backpressure of the blower) the change in flow between inspiration andexpiration can be relatively large, for example 5 L/min or more. Thechange will be greater when the user set flow is relatively low.Controlling flow during breathing is generally more difficult thancontrolling average flow. This is because the time response of the motorused as part of the blower unit 13 is often comparable to breath rate.Care needs to be taken to ensure that the breathing system such as thebreathing assistance system 1 will be stable at all operatingconditions, but maintains a sufficiently fast response. This is done bycareful choice of the control parameters. For example if a P.I.D. systemis used the P, I and D gains must be set very carefully.

The intra-breath control method is implemented in the preferred form asfollows, with reference to FIG. 10 a.

Firstly, the flow is sampled at a rate that allows intra breathvariations to be picked up. In the preferred embodiment, this samplerate is in the region of 25 Hz (e.g. 20-30 Hz—that is, the flow rate ismeasured by the flow probe 61 (or 161 or 261) between 20 and 30 timesper second). The flow probe 61 used in the preferred form of breathingassistance system 1 must be able to respond to changes sufficientlyquickly to achieve this response. As outlined above, P.I.D. controlalgorithms are pre-loaded for use in the controller 8. A problem withthe ‘D’ or Derivative term 303 a or 303 b is that small amounts ofmeasurement or process noise can cause large amounts of change in theoutput. In the preferred form of the present invention, in order toensure the response is sufficiently rapid, this filter is not present.Alternatively, as shown in FIG. 10a , a low pass filter 321 with cut-offfrequency high enough to allow intra-breath flow variation to passunattenuated or nearly unattenuated is used. This increases the responsetime of the fine control system so that both the average and the intrabreath variation will be compensated for. Care needs to be taken toensure that the parameters of the control filter are chosen to ensureunwanted effects such as overshoot and oscillation that will cause theuser discomfort do not occur over the entire range of flows used and forall patient interfaces used.

The system could also be used without the filter 321 present. However,removing this filter may require the use of a more accurate flow sensor.The gains used will have to be kept small enough to make sure that thenoise does not adversely affect behaviour—this may result in aperformance that is not ideal, e.g. the flow may not be as constant asone would like.

As outlined above, the controller 8 uses either fine or coarse controlby constantly receiving input from the flow probe 61, which samples theflow rate between 20 and 30 times per second in the preferredembodiment. The instantaneous flow is used to calculate the average flowover a period of time greater than a few breath cycles using e.g. a lowpass filter 320 which is used to calculate the deviation of the averageflow from the user-set or desired flow. In the preferred embodiment, ifthe measured average flow is different by a preset value of e.g. greaterthan 3 Litres/minute from the user-set or desired flow rate, then thecontroller 8 uses coarse control parameters or ‘large flow deviations’313 a to adjust the flow rate to the user-set level. If the average flowrate deviates from the average by a proportion of 15%, or more than 3litres/minute, then the controller 8 or 108 initiates coarse control.Otherwise, fine control or small flow deviations 313 b are used.

In order to ensure that stable operation is maintained during coarsecontrol the average flow obtained using the output of filter 320 can befed back into the controller rather than the instantaneous measured flowshown in FIG. 10 a.

In a variant or second preferred form or embodiment, the controller 8compensates for flow variation resulting from the breathing cycle bypassing the signal 307 a (the signal indicative of actual flow rate) inparallel through a low pass filter 308 and a high pass filter 309, asshown in FIG. 10b . The low pass filter produces an output signal 307 b.The high pass filter 309 produces an output signal 315 that feeds backto a compensation filter 306. The output signal 311 from the P.I.Dcontroller and the output signal 312 a from the compensation filter 306is used to control the speed of the fan in the fan unit 13. This has theadvantage of allowing the P.I.D. filter 313 for the average to be setindependently of the intra-breath control filter. This makes it easierto design a stable and robust control system.

The dual feedback loops shown in FIG. 10b allow separate P.I.D. gains,so that average and intra breath flow can be controlled. The decision asto whether to use fine or coarse control for the adjustment of the meanflow is made by examining the deviation of the output of the low passfilter, 307 b, from the user set flow as described previously.

Yet another difficulty which is encountered with prior art systems, isthat the breathing assistance system is a nonlinear system—the open loopgain to the system varies with the state of the breathing assistancesystem. That is, a given change in blower pressure or motor speed willproduce a change in the flow rate that depends on the current state ofthe breathing assistance system. For example if the blower unit 3 isoperating at a high flow rate condition, and the overall flow ratechanges by a certain amount because the user exhales, the change inpressure or motor speed required to compensate for this change will bedifferent than it would be if the blower unit 3 was operating at a lowflow rate. This can cause problems with stability, and it is possiblefor prior art control systems to become unstable at some flow values ormotor speeds. Also it is possible that the response time may become tooslow to adequately compensate for intra-breath variation. This can be aparticularly problematic in a system where the response time is similarto that of the disturbance, for example in systems where rate of flowvariation is similar to the time response of the fan unit 13.

There are a variety of different controllers that can be modified tohelp overcome these effects. One way is to use a controller with acontrol filter with parameters that vary as a function of the state ofthe system. For example, if a P.I.D. controller is used the P, I and Dparameters may not be constant but a function of the average (or eveninstantaneous) flow, or blower pressure or motor speed or of the userset flow.

FIG. 11 shows a schematic diagram of how this might be achieved. Thecontrol system is the same as that shown in FIG. 10 and as describedabove, but with the addition of a feedback signal 316 from the flowgenerator or fan unit 13 to the compensation filter 306. The inputsignal to the fan unit 13 in this variant will therefore be the outputsignal 311 from the P.I.D. filter 313, and the signal 312 b from thecompensation filter 306.

What is claimed is:
 1. A breathing assistance system configured todeliver a stream of gases for therapeutic purposes, comprising: ahumidifier including an inlet port and an exit port, said humidifierconfigured to receive a flow of gases from a gases source via said inletport and hold and heat a volume of water, said flow of gases passingthrough said humidifier, becoming heated and humidified, and exitingsaid humidifier via said exit port; a delivery conduit configured toreceive said flow of gases from said exit port for delivery to a uservia an interface, said delivery conduit including a heater wireconfigured to heat the flow of gases within said delivery conduit; apatient end temperature sensor adapted to measure a first temperaturerelating to said flow of gases at or close to said user; an exit porttemperature sensor configured to measure a second temperature relatingto said flow of gases exiting said humidifier; an ambient temperaturesensor adapted to measure a third temperature relating to said flow ofgases before said flow of gases enters said humidifier; a flow sensorconfigured to measure a flow rate of said flow of gases; and acontroller configured to: receive data from said patient end temperaturesensor relating to the first temperature, data from said exit porttemperature sensor relating to the second temperature, data from saidambient temperature sensor relating to the third temperature, and datafrom said flow sensor relating to the flow rate, based on said datarelating to the second and third temperatures and said data relating tothe flow rate, determine a first control output for adjusting powerprovided to said humidifier to achieve a first desired output at saidexit port, and based in said data relating to the first temperature andsaid data relating to the flow rate, determine a second control outputfor adjusting power provided to said heater wire to maintain or alter atemperature of said flow of gases within said delivery conduit toachieve a second desired output at said interface.
 2. The breathingassistance system as claimed in claim 1, wherein said first controloutput relates to a target temperature at said exit port for a givenflow level, said first desired output comprises a target temperature atsaid exit port, and said first control output adjusts said powerprovided to said humidifier to match said second temperature with saidtarget temperature at said exit port.
 3. The breathing assistance systemas claimed in claim 1, wherein said second control output relates to atarget temperature at said interface for a given flow level, and saidsecond control output adjusts said power provided to said heater wire tomatch said first temperature with said target temperature at saidinterface.
 4. The breathing assistance system as claimed in claim 1,wherein the controller is configured to determine at least one of saidfirst control output or second control output based at least in part ona rule-based system loaded in a memory of said controller, amathematical formula loaded in the memory, or a look-up table loaded inthe memory.
 5. The breathing assistance system as claimed in claim 1,wherein at least one of said first or second desired output comprises auser-set target dew point temperature.
 6. The breathing assistancesystem as claimed in claim 5, wherein said user-set target dew pointtemperature relates to an absolute humidity level of substantially 44 mgH2O/liter of air.
 7. The breathing assistance system as claimed in claim1, wherein at least one of a target temperature at said interface or atarget temperature at said exit port is in the range of 31-39° C.
 8. Thebreathing assistance system as claimed in claim 1, wherein at least oneof said first or second desired output comprises a target absolutehumidity.
 9. The breathing assistance system as claimed in claim 1,wherein at least one of said first or second desired output comprises atarget temperature and relative humidity.
 10. The breathing assistancesystem as claimed in claim 1, further comprising one or more usercontrols configured to enable said user to set a desired user-set flowrate of gases.
 11. The breathing assistance system as claimed in claim10, further comprising a flow controller configured to receive said flowof gases from a remote central source, said flow controller locatedbetween said remote central source and said humidifier, said flowcontroller receiving said flow of gases and passing said flow of gaseson to said humidifier via a gases connection path between saidhumidifier and said flow controller, said one or more user controlsconfigured to enable said user to set said desired user-set flow rate.12. The breathing assistance system as claimed in claim 11, wherein saidflow controller further comprises a venturi adapted to mix said flow ofgases from said remote central source with atmospheric gases beforepassing said flow of gases to said humidifier.
 13. The breathingassistance system as claimed in claim 1, wherein said gases sourcecomprises a blower configured to be fluidically connected to saidhumidifier, said blower including an adjustable, variable speed fan unitconfigured to deliver said flow of gases over a range of flow rates tosaid humidifier and one or more user controls configured to enable saiduser to set a desired user-set flow rate, said controller configured tocontrol power provided to said blower to produce said desired user-setflow rate.
 14. The breathing assistance system as claimed in claim 1,wherein said humidifier comprises a humidifier chamber including aheater base, and said breathing assistance system further comprises: aheater plate adapted to heat contents of said humidifier chamber byproviding energy to said heater base; and a heater plate temperaturesensor adapted to measure a fourth temperature relating to said heaterplate, said controller further configured to determine said firstcontrol output by assessing the second, third, and fourth temperaturesand said flow rate and adjusting at least power provided to said heaterplate to match said second temperature to a target temperature at saidexit port.
 15. The breathing assistance system as claimed in claim 1,further comprising a humidity sensor configured to measure a humidity ofatmospheric gases entering said humidifier, said controller configuredto determine at least one of said first control output or said secondcontrol output further based on data relating to the measured humidity.16. The breathing assistance system as claimed in claim 1, wherein thecontroller is configured to determine said second control output furtherbased on said data related to the third temperature.
 17. A method ofoperating a breathing assistance system that delivers a stream of gasesfor therapeutic purposes, the method comprising: under control of acontroller of the breathing assistance system: determining a firsttemperature relating to a flow of gases at or close to a user, said flowof gases configured to enter a humidifier of the breathing assistancesystem via an inlet port, become heated and humidified, and exit saidhumidifier via an exit port, said flow of gases configured to bedelivered to a user via an interface, said interface configured to beconnected to said humidifier by a delivery conduit including a heaterwire configured to heat the flow of gases within said delivery conduit;determining a second temperature relating to said flow of gases exitingsaid humidifier; determining a third temperature relating to said flowof gases before said flow of gases enters said humidifier; determining aflow rate of said flow of gases; based on the second and thirdtemperatures and the flow rate, determining a first control output foradjusting power provided to said humidifier to achieve a first desiredoutput at said exit port; and based on the first temperature and theflow rate, determining a second control output for adjusting powerprovided to said heater wire to maintain or alter a temperature of saidflow of gases within said delivery conduit to achieve a second desiredoutput at said interface.
 18. The method as claimed in claim 17, whereinsaid first control output relates to a target temperature at said exitport for a given flow level, said first desired output comprises atarget temperature at said exit port, and said first control outputadjusts said power provided to said humidifier to match said secondtemperature with said target temperature at said exit port.
 19. Themethod as claimed in claim 17, wherein said second control outputrelates to a target temperature at said interface for a given flowlevel, and said second control output adjusts said power provided tosaid heater wire to match said first temperature with said targettemperature at said interface.
 20. The method as claimed in claim 17,wherein determining at least one of said first control output or secondcontrol output is based at least in part on a rule-based system loadedin a memory of said controller, a mathematical formula loaded in thememory, or a look-up table loaded in the memory.
 21. The method asclaimed in claim 17, wherein at least one of said first or seconddesired output comprises a target temperature and relative humidity.